It is well-known that during the erection of an offshore or onshore wind turbine the tower sections, the nacelle, the rotor hub, and the wind turbine blades are turned and lifted into position by using two crane units. The various parts can be lifted into position individually or be pre-assembled at an installation site, at a loading site or at the deck before being lifted into position. It is known that at least two of the wind turbine blades or hub extenders can be mounted to the rotor hub before lifting the rotor into position. The rotor hub is normally transported to the installation site in a vertical position and then turned into a horizontal position using a second smaller crane unit in order to allow mounting to the nacelle. The larger crane unit is normally secured to the rotor by using lifting brackets mounted to two of the pitch bearings located on the rotor hub. The lifting brackets are removed before the wind turbine blades are mounted.
The rotor hub can instead be placed in a temporary support frame and then rotated into the horizontal position around a rotation axis in the support frame. However, this solution adds to the total installation costs and introduces additional steps in the assembly process.
US 2009/0324380 A1 discloses a lifting device for lifting and turning a rotor hub into position relative to a nacelle. A mounting end of the lifting device is extended into one of the openings for the wind turbine blades and mounted to an interior structural part of the rotor hub, such as the pitch bearing. A crane wire is secured to the free end of a rotatable arm extending out of the opening. A hydraulic actuator is remotely activated during the lifting process to rotate the arm and the rotor hub around a rotation point. A second crane is needed to control the movement of the rotor hub in order to prevent it from turning and potentially damaging the truck bed during the initial phase of the lifting step. Furthermore, this lifting device requires that at least one wind turbine blade is lifted into position and installed afterwards as the lifting device occupies one of the openings.
US 2013/0269188 A1 discloses a similar lifting device where a mounting end is extended into one of the openings and mounted to an inner surface of the respective pitch bearing. A crane wire is secured to the other end of the lifting device and a temporary pitch control unit is connected to the pitching mechanism of the rotor hub via electrical cables or hydraulic hoses. The rotor hub is then lifted out of its support frame and the pitching mechanism of the rotor hub is activated to rotate the rotor hub around the centre line of that pitch bearing. The pitch control unit is then disconnected and the rotor hub is lifted into position relative to the nacelle. The lifting device takes up one of the openings which means that one wind turbine blade has to be mounted afterwards. Secondly, the use of a separate pitch control unit adds to the costs of the lifting device and requires a worker to manually connect and disconnect the pitch control unit each time the lifting device is used.
US 2009/0159549 A1 discloses a lifting device mounted to the outer surface of the rotor hub. In one embodiment, the lifting device has a mounting frame to which an actuator and a very large and heavy lifting arm are pivotal connected. The crane wire is connected to connection means at the opposite end of the lifting arm. Both the lifting arm and the actuator extend outwards from the frame, even in the compact state, meaning that these elements and their connection points are subjected to considerable transverse moments during the lift. In another embodiment, the lifting device has a curve track on which a trolley assembly is located. The crane wire is connected to the trolley and a gear inside the trolley is used to rotate the rollers which move the trolley along the track. The trolley can only be positioned at the end positions as no braking mechanism is provided.